A typical aircraft has landing gear comprising a plurality of undercarriages which support the aircraft when it is on the ground. The undercarriages are used to control the movement of the aircraft during ground maneuvers such as landing, taxiing and take off. Some of the undercarriages have braking wheels which are operable to provide a braking force to decelerate the aircraft when a braking torque is applied by a set of brakes. At least one of the undercarriages has a steering wheel which may be rotated to steer the aircraft.
The ability of the aircraft to perform certain ground maneuvers may be limited by the performance envelope of the undercarriages. For example, the ability of the aircraft to perform a braking operation or a steering operation may be limited by the maximum braking force which can be generated by the braking wheels (and by other braking systems, e.g. reverse thrust of the aircraft engines) or the maximum turning moment which can be generated by the steering wheels (and by other steering systems, e.g. differential thrust of the aircraft engines).
The performance envelope of the undercarriages may vary depending on the operating conditions of the undercarriages, for example due to variations in brake gain (the ratio of the brake clamping force to the resultant braking torque), changes in temperature or loading conditions, as well as external conditions such as adverse runway conditions, variations in runway surface and crosswinds. The operability of certain components and systems may also affect the performance envelope. For example, performance may be reduced in the case of tire burst or brake failure or if certain components become excessively worn. It is therefore difficult for a pilot or a control system to determine how to operate the braking and steering systems most efficiently during ground maneuvers.
During ground maneuvers, aircraft undercarriages may experience spike loads significantly greater than those expected during the majority of normal operation of the aircraft. Aircraft undercarriages are generally designed conservatively to withstand these increased loads, leading to a significant increase in aircraft weight and consequently a reduction in fuel efficiency. It is considered desirable to control and limit the loads generated in aircraft undercarriages so that the structural efficiency may be increased, allowing the weight and therefore fuel consumption of the aircraft to be reduced.
It is therefore desirable to provide a steering system for an aircraft which addresses these problems and allows an aircraft to perform ground maneuvers with greater efficiency within the available performance envelope.